Methods and apparatuses for measuring frequencies of basestations in cellular networks using mobile GPS receivers

ABSTRACT

Methods and systems for frequency synchronizing base stations in a cellular communication system. In one aspect of the invention, a method to measure a frequency related to a base station of a cellular communication system includes receiving a satellite positioning system signal; determining a frequency of a reference signal from a local oscillator of a mobile station from the satellite position system signal; receiving a first cellular signal from a base station containing first and second timing markers; determining first and second time tags for the markers using the reference signal; and combining the frequency of the reference signal and the first and second time tags to compute a first frequency related to the base station.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/189,044, filed on Jul. 2, 2002 now U.S. Pat. No. 6,937,872, thisapplication also claims priority to U.S. Provisional Application No.60/372,944, filed on Apr. 15, 2002.

FIELD OF THE INVENTION

The present invention relates to the field of cellular communicationsystems, and particularly to those systems where the location of amobile cellular communication station (MS) is determined.

BACKGROUND OF THE INVENTION

To perform position location in cellular networks (e.g., a cellulartelephone network), several approaches perform triangulation based uponthe use of timing information sent between each of several basestationsand a mobile device, such as a cellular telephone. In one approach,called Time Difference of Arrival (TDOA), the times of reception of asignal from a mobile device is measured at several basestations, andthese times are transmitted to a location determination entity, called alocation server, which computes the position of the mobile device usingthese times of reception. For this approach to work, the accuratepositions of the basestations need to be known, and the times-of-day atthese basestations need to be coordinated in order to provide anaccurate measurement of the location. Time coordination is an operationto keep, at a particular instance of time, the times of day associatedwith multiple basestations within a specified error bound.

FIG. 1 shows an example of a TDOA system where the times of reception(TR1, TR2, and TR3) of the same signal from the mobile cellulartelephone 111 are measured at cellular basestations 101, 103, and 105and processed by a location server 115. The location server 115 iscoupled to receive data from the basestations through the mobileswitching center 113. The mobile switching center 113 provides signals(e.g., voice communications) to and from the land-line Public SwitchedTelephone System (PSTS) so that signals may be conveyed to and from themobile telephone to other telephones (e.g., land-line phones on the PSTSor other mobile telephones). In some cases the location server may alsocommunicate with the mobile switching center via a cellular link. Thelocation server may also monitor emissions from several of thebasestations in an effort to determine the relative timing of theseemissions.

An alternative method, called Enhanced Observed Time Difference (EOTD)or Advanced Forward Link Trilateration (AFLT), measures at the mobiledevice the times of arrival of signals transmitted from each of severalbasestations. FIG. 1 applies to this case if the arrows of TR1, TR2, andTR3 are reversed. This timing data may then be used to compute theposition of the mobile device. Such computation may be done at themobile device itself, or at a location server if the timing informationso obtained by the mobile device is transmitted to the location servervia a communication link. Again, the times-of-day of the basestationsmust be coordinated, and their locations accurately assessed. In eitherapproach, the locations of the basestations may be determined bystandard surveying methods and be stored in the basestations, at thelocation server, or elsewhere in the network in some type of computermemory.

Yet a third method of doing position location involves the use in themobile device of a receiver for the Global Positioning Satellite System(GPS) or other satellite positioning system (SPS). Such a method may becompletely autonomous or may utilize the cellular network to provideassistance data or to share in the position calculation. Examples ofsuch a method are described in U.S. Pat. No. 5,841,396; No. 5,945,944;and No. 5,812,087. As a shorthand, we call these various methods “SPS”.In practical low-cost implementations, both the mobile cellularcommunications receiver and the SPS receiver are integrated into thesame enclosure and, may in fact share common electronic circuitry.

A combination of either the EOTD or TDOA with an SPS system is called a“hybrid” system.

It should be clear from the above description that, for EOTD, TDOA, orhybrid systems, time coordination between the various cellularbasestations is necessary for accurate position calculation of themobile device. The required accuracy of the times-of-day at thebasestations depends upon the details of the positioning methodutilized.

In yet another variation of the above methods, the round trip delay(RTD) is found for signals that are sent from the basestation to themobile device and then are returned. In a similar, but alternative,method the round trip delay is found for signals that are sent from themobile device to the basestation and then returned. Each of theseround-trip delays is divided by two to determine an estimate of theone-way time delay. Knowledge of the location of the basestation, plus aone-way delay constrains the location of the mobile device to a circleon the earth. Two such measurements then result in the intersection oftwo circles, which in turn constrains the location to two points on theearth. A third measurement (even an angle of arrival or cell sector)resolves the ambiguity. With the round trip delay approach, it isimportant that the RTD measurements be coordinated to be taken withinseveral seconds, at worst, so that if the mobile device is movingrapidly, the measurements correspond to the mobile device being near thesame location.

In many situations, it is not possible to perform round tripmeasurements to each of two or three basestations, but only to onebasestation, which is the primary one communicating with the mobiledevice. For example, this is the case when the IS-95 North American CDMAcellular standard is used. Or it may not be possible to perform accurate(e.g., submicrosecond) round trip timing measurements at all due toequipment or signaling protocol limitations. This appears to be the casewhen the GSM cellular communication standard is used. In these cases, itis even more important that accurate timing (or relative timing) bemaintained on the basestation transmissions if a triangulation operationis to be performed, since only the time differences between differentmobile-basestation paths are utilized.

Another reason to maintain accurate timing information at basestationsis to provide time to the mobile devices for aiding GPS based positioncalculations; and such information may result in reduced time to firstfix, and/or improved sensitivity. U.S. Pat. No. 6,150,980 and No.6,052,081 contain such examples. The required accuracy for thesesituations can range from a few microseconds to around 10 milliseconds,depending upon the performance improvement desired. In a hybrid system,the basestation timing serves the dual purpose of improving the TDOA (orEOTD) operation as well as the GPS operation.

The prior art approaches to basestation timing coordination employspecial fixed location timing systems, termed Location Measurement Units(LMU) or Timing Measurement Units (TMU). These units typically includefixed location GPS receivers which enable the determination of accuratetime-of-day. The location of the units may be surveyed, such as may bedone with GPS based surveying equipment. In alternative implementations,the LMUs or TMUs may not rely upon an absolute time provided by a GPSreceiver or other source, but may simply relate the timing of onebasestation versus that of another basestation, in a differential sense.However, such an alternative approach (without using a GPS receiver)relies upon the observability of multiple basestations by a singleentity. Furthermore, such an approach may give rise to cumulative errorsacross a network.

Typically, LMUs or TMUs observe the timing signals, such as framingmarkers, present within the cellular communication signals that aretransmitted from the basestations and attempt to time-tag these timingsignals with the local time found via a GPS set or other timedetermination device. Messages may subsequently be sent to thebasestations (or other infrastructure components), which allow theseentities to keep track of elapsed time. Then, upon command, orperiodically, special messages may be sent over the cellular network tomobile devices served by the network indicating the time-of-dayassociated with the framing structure of the signal. This isparticularly easy for a system such as GSM in which the total framingstructure lasts over a period exceeding 3 hours. Note that the locationmeasurement units may serve other purposes, such as acting as thelocation servers—that is, the LMUs may actually perform thetime-of-arrival measurements from the mobile devices in order todetermine the position of the mobile devices.

One problem with these LMU or TMU approach is that they require theconstruction of new special fixed equipment at each basestation or atother sites within communication range of several basestations. This canlead to very high costs for installation and maintenance.

SUMMARY OF THE INVENTION

Methods and apparatuses for frequency synchronizing basestations in acellular communication system are described here.

In one aspect of the invention, a method to predict a timing oftransmission of a basestation in a cellular communication systemincludes: receiving a first time tag for a first timing marker in afirst cellular signal transmitted from the basestation; receiving asecond time tag of a second timing marker in a second cellular signaltransmitted from the basestation; and computing a frequency related tothe basestation using the first and second time tags. Each of the timetags are determined using at least one satellite positioning systemsignal received at a mobile station, which also receives thecorresponding time marker contained in the cellular signal from thebasestation. In one example according to this aspect, the time tags aredetermined from the time-of-day messages in satellite positioningsignals. In another example according to this aspect, the timedifference between at least two time tags are determined from localreference signals, the frequencies of which are determined from theprocessing of satellite positioning signals.

In another aspect of the invention, a method to measure a frequencyrelated to a basestation includes: receiving, at a mobile station, atleast one satellite positioning system signal; determining a frequencyof a reference signal from a local oscillator of the mobile station fromthe at least one satellite positioning system signal; receiving, at themobile station, a cellular signal from the basestation, the cellularsignal being modulated upon a carrier; measuring a frequency of thecarrier using the reference signal from the local oscillator; anddetermining a frequency related to the basestation using the frequencyof the carrier.

The present invention includes apparatuses which perform these methods,including data processing systems which perform these methods andmachine readable media which when executed on data processing systemcause the systems to perform these methods.

Other features of the present invention will be apparent from theaccompanying drawings and from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 shows an example of a prior art cellular network which determinesthe position of a mobile cellular device.

FIG. 2 shows an example of a mobile cellular communication station whichmay be used with the present invention and which includes a GPS receiverand a cellular communication transceiver.

FIG. 3 shows a block diagram representation of a combined mobile stationwhich may be used with the present invention and which shares the commoncircuitry between a GPS receiver and a cellular communicationtransceiver.

FIG. 4 shows an example of a cellular basestation which may be used invarious embodiments of the present invention.

FIG. 5 shows an example of a server which may be used with the presentinvention.

FIG. 6 illustrates a network topology for measuring frequencies ofbasestation signals according to one embodiment of the presentinvention.

FIG. 7 shows the framing structure of GSM cellular signals.

FIG. 8 shows a flow chart for determining a frequency of a basestationaccording to one embodiment of the present invention.

FIG. 9 shows a detailed method to determine a frequency of basestationsignals by using measurements of framing epochs of the basestationsignals according to one embodiment of the present invention.

FIG. 10 shows another method to determine a frequency of basestationsignals by using measurements of framing epochs of the basestationsignals according to one embodiment of the present invention.

FIG. 11 shows a detailed method to determine a frequency of basestationsignals by using measurements of carrier frequency of the basestationsignals according to one embodiment of the present invention.

DETAILED DESCRIPTION

The following description and drawings are illustrative of the inventionand are not to be construed as limiting the invention. Numerous specificdetails are described to provide a thorough understanding of the presentinvention. However, in certain instances, well known or conventionaldetails are not described in order to avoid obscuring the description ofthe present invention.

In most digital cellular systems, numbered framing markers aretransmitted as part of the cellular system transmissions. In a networksuch as GSM, the time-of-day information from a GPS receiver may be usedto time tag the framing structure (e.g., framing markers) of thereceived communication (e.g., GSM) signal. For example, the start of aparticular GSM frame boundary, which occurs every 4.6 milliseconds, maybe used (see FIG. 7). There are 2,715,648 such frames per hyperframe,which last 3.48 hours; hence each such frame boundary is unambiguous forall practical purposes. Co-pending U.S. patent application Ser. No.09/565,212, filed on May 4, 2000, describes a method for timecoordination, in which mobile stations (MS) containing GPS receivers areutilized to measure both time-of-day and position to a high accuracy.The time tag information of the cellular framing structure measured atthe mobile station is passed via normal cellular signaling to thebasestation (BS) (e.g., a cellular basestation shown in FIG. 4), orother network entity (e.g., a server or a location server), to determinethe time-of-day of the basestation. The delay due to the propagationtime from the mobile station (MS) (e.g., the mobile cellularcommunication station shown in FIG. 2) to the basestation (BS) can bedetermined (typically at the basestation or other network entity) bydividing the BS-MS range by the speed of light, since the mobile stationhas determined its position via the GPS unit and the basestation knowsits accurate location (e.g., via a survey). Then the basestation maydetermine the timing of its transmitted frame marker by simplysubtracting the computed propagation time from the time tag of theframing marker provided by the mobile station.

Closely related to time coordination between basestations is frequencycoordination (or synchronization) between basestations. Onceestablished, it is desirable that coordination in time be maintainedover a long period of time. Otherwise such time coordination would haveto be performed often, which might be a complex and expensive operation.For example, basestations might coordinate their time by sending signalsback and forth between themselves over existing communication channels(e.g., cellular channels). If such signaling is required on a continuousbasis, valuable communication resources are wasted, which couldotherwise be employed for transmitting other voice and data information.

To avoid frequent time coordination, it is desirable to have at eachbasestation an accurate measurement of the frequency of the primarysignal source, or alternatively, the frequency of the basestation'ssource relative to those of other basestations. If the frequencies ofthe primary signal sources of the basestations are known to a highaccuracy, the times of day at these basestations, once coordinated, canbe maintained for a long period of time by utilizing time-intervalcounters.

At least one embodiment of the present invention seeks to performfrequency coordination between basestations. The methods according tothe present invention utilize normal mobile cellular communicationreceivers that are equipped with GPS positioning capability, withoutdeploying fixed and expensive network resources.

One embodiment of the present invention utilizes cellular transmissiontiming markers (e.g., framing markers) for frequency synchronization.Measurements of the basestation framing marker transmission frequenciesis used to provide a precise estimate of the error between the optimaland the true timing between successive framing markers. This error maybe propagated forward in time as a function of the marker number byutilizing a standard curve fit type algorithm. Thus, the frame markertimes-of-occurrence may be used as an accurate clock for a long periodof time once an initial frame marker timing is ascertained and a goodestimate of the frame marker rate (or error from the nominal rate) isascertained.

Another embodiment of the present invention utilizes the carrierfrequency of cellular transmissions for frequency synchronization. Inmost cases both the framing markers and the carrier frequency of acellular signal from a basestation are synchronized to the samereference signal generator at the basestation. Hence, by simplemathematical calculation, the frequency of the framing marker of abasestation signal can be ascertained from the carrier frequency of thecellular signal.

In at least one embodiment of the present invention, the frequency ofthe framing marker transmitted by the cellular basestation transmitteris determined for frequency coordination. However, the framing markers,and the signal symbols (assuming digital modulation), as well as thesignal carrier frequency, are normally all synchronized to one commonmaster oscillator (e.g., oscillator 413 in FIG. 4) in a digital cellularsystem. In several important cellular systems, including the GSM system,the Japanese PDC system and the WCDMA system, the frequency of thetiming signals (e.g., framing marker) and the carrier frequency arederived from the same basic oscillator. Hence, precise measurements ofeither the rate of transmission of timing markers (the symbol rate) orthe carrier frequency of such transmissions can be used to accomplishthe same goal. The carrier frequency may be used to infer the frequencyof transmission and vice versa. The advantages and disadvantages inmeasuring either of them are related to the details of theimplementations and measurement accuracy.

In one embodiment, one or more mobile stations make one or more timingmeasurements of received basestation signals and transmit these timetags and optional additional information to a server, which in turnperforms a frequency calculation.

In another embodiment, one or more mobile stations measure the carrierfrequency of received basestation signals and transmit the informationabout the carrier frequency and optional additional information to aserver.

In another embodiment, one or more mobile stations each makes at leasttwo timing measurements for the received basestation signals, computes afrequency (or, equivalently a time interval) measure based upon thesemeasurements, and transmits the frequency measure to a server.

In various embodiments, the server may collect a succession of data fromthe mobile stations to perform further processing for a betterestimation of the frequency, or to perform a curve fit operation uponsuch frequency versus time information.

It will be appreciated that the cellular basestation transmissionfrequency may be computed at a basestation (BS), or at a mobile station(MS), or a server (e.g., a location server or other network entities).

Thus, to time synchronize basestations (equivalently, to determine themarker timings of emissions from these basestations), various methodsaccording to the present invention determine the frequency of suchemissions from the basestations, which can be an important part of thetime synchronization problem, as described previously. The details ofthe methods are described below.

FIG. 2 shows an example of a mobile station containing a GPS receiver,which may be used with the present invention. The GPS receiver candetermine the, time-of-day at the instance of receiving a signal (e.g.,a timing marker of a cellular signal received at transceiver 213) andthe position of the receiver, as well as the frequency of an externallysupplied signal, to a high accuracy. The measurements of thetime-of-day, position, and frequency may be done in an autonomous modeif the level of the received signal is high, or with the aid ofequipment in the infrastructure (servers) if the signal-to-noise ratioof the received signal is low (e.g., see U.S. Pat. No. 5,945,944; No.5,841,396; and No. 5,812,087).

The mobile cellular communication station 210 shown in FIG. 2 includes aGPS receiver 211 connecting to a GPS antenna 203 and a cellularcommunication transceiver 213 connecting to a communication antenna 201.Alternatively, GPS receiver 211 may be contained within another chassis;in this situation, the station 210 does not include a GPS receiver nordoes it require one, as long as the GPS receiver is coupled to and isco-located with the station 210.

The GPS receiver 211 may be a conventional, hardware correlator basedGPS receiver, or it may be a matched filter based GPS receiver, or itmay be a GPS receiver which uses a buffer to store digitized GPS signalswhich are processed with fast convolutions, or it may be a GPS receiveras described in U.S. Pat. No. 6,002,363 in which the components of theGPS receiver are shared with the components of the cellularcommunication transceiver (e.g., see FIG. 7B of U.S. Pat. No. 6,002,363which is hereby incorporated here by reference).

The cellular communication transceiver 213 may be a modem cellulartelephone which operates with any one of the well-known cellularstandards, including: the GSM cellular standard, or the Japanese PDCcommunication standard, or the Japanese PHS communication standard, orthe AMPS analog communication standard, or the North American IS-136communication standard, or the unsynchronized wideband spread spectrumCDMA standard.

The GPS receiver 211 is coupled to the cellular communicationtransceiver 213 to provide GPS time and position in one embodiment tothe cellular communication transceiver 213 (which then transmits thisinformation to a basestation). In another embodiment, GPS receiver 211provides aiding in the precise measurement of the carrier frequency ofthe cellular signal received by transceiver 213.

In one embodiment GPS time may be obtained at the mobile station 210 byreading GPS time off the GPS signals from the GPS satellites.Alternatively, a technique for determining time as described in U.S.Pat. No. 5,812,087 may be utilized. In this approach, a sample of theGPS signals received at the mobile may be transmitted to a locationserver or to some other servers where the signal sample is processed todetermine the time of receipt as described in U.S. Pat. No. 5,812,087.Further, the time-of-day may be alternatively be computed using one ofthe various methods described in U.S. Pat. No. 6,215,442.

Furthermore, the cellular communication transceiver 213 may provideassistance data such as Doppler information or time information to theGPS receiver as described in U.S. Pat. No. 5,841,396, and No. 5,945,944.The coupling between the GPS receiver 211 and the cellular communicationtransceiver 213 may also be utilized to transmit a record of GPS data toor from a cellular basestation for the purpose of matching that recordwith another record in order to determine the time at the GPS receiver,as described in U.S. Pat. No. 5,812,087. In those situation orembodiments where a location server is used to provide assistance datato the mobile cellular communication station for the purpose ofdetermining the position or time at the system 210, or a location servershares in the processing of information (e.g., the location serverdetermines time or the final position calculation of the mobile system210), it will be appreciated that a location server such as that shownin FIG. 5 and described further below is connected to a cellularbasestation through a communication link to assist in the processing ofdata.

The position of the mobile station 210 is normally not fixed and isnormally not predetermined.

FIG. 3 shows a block diagram representation of a combined mobile stationwhich may be used with the present invention and which shares the commoncircuitry between a GPS receiver and a cellular communicationtransceiver. The combined mobile station 310 includes circuitry forperforming the functions required for processing GPS signals as well asthe functions required for processing communication signals receivedthrough a cellular communication link 360 to or from a basestation 352.

Mobile station 310 is a combined GPS receiver and a cellularcommunication transceiver. Acquisition and tracking circuit 321 iscoupled to GPS antenna 301, and communication transceiver 305 is coupledto communication antenna 311. Oscillator 323 provides reference signalsto both circuit 321 and communication receiver 332. GPS signals arereceived through GPS antenna 301 and input to circuit 321 which acquiresGPS signals received from various satellites. Processor 333 processesthe data produced by circuit 321 for transmittal by transceiver 305.Communication transceiver 305 contains a transmit/receive switch 331which routes communication signals (typically RF) to and fromcommunication antenna 311. In some systems, a band splitting filter, or“duplexer,” is used instead of the T/R switch. Received communicationsignals are input to communication transceiver 305 and passed toprocessor 333 for processing. Communication signals to be transmittedfrom processor 333 are propagated to modulator 334, frequency converter335, and power amplifier 336. U.S. Pat. No. 5,874,914, herebyincorporated here by reference, describes details about combined mobilestation that contains a GPS receiver and a cellular transceiver andutilizes a communication link.

The carrier frequency of a cellular signal from a basestation may bemeasured using a GPS receiver in a variety of ways. In one method,cellular receiver 332 frequency locks or phase locks to the receivedcarrier from the basestation. This is typically done with the aid of avoltage controlled oscillator (VCO) (e.g., oscillator 323) in aphase-locked or frequency-locked loop configuration, which may becontrolled by a signal from the communication receiver on line 340. Thelong term frequency of the VCO is then proportional to that of thebasestation's transmitted carrier frequency (after removing a Dopplerfrequency offset due to the velocity of the mobile station). The VCOoutput may then be used as a frequency reference for the GPS receiver'sdownconverter circuitry (e.g., that used by acquisition and trackingcircuit 321). As part of the signal processing in the GPS receiver,frequency errors are determined for the various received GPS signalsreceived from several GPS satellites. Each such received signal willalso contain a common component of such frequency errors due to the VCOerror relative to an idealized value. This frequency error due to theVCO (a so-called “bias” frequency) may then be determined and scaled todetermine the basestation frequency, after the Doppler induced frequencyoffset, due to the motion of the mobile station, is removed.

It is well known that such “common mode” frequency biases can beobtained in GPS processing. Received frequency errors are due to acombination of receiver motion and to the common mode bias. User motionis described by a three-component velocity vector. Hence, including thecommon mode bias, there are basically four frequency related unknownsfor to be solved for. Signals received from four different GPSsatellites will normally allow the solution of these four equations andhence the common mode bias error due to the VCO error. Performingmultiple sets of frequency measurements over a period of time canfurther reduce the number of GPS satellite signals that must bereceived. Likewise, constraining the receiver velocity (e.g. assumingthat there is little z-axis motion), can further reduce the number ofrequired received satellite signals.

As an alternative to the above approach, a GPS receiver may have areference signal that is independent from the VCO used by the cellulartransceiver. In this case, the GPS receiver again determines thefrequency of its reference signal (typically from a crystal oscillator).The output of the cellular transceiver VCO and the reference signal forthe GPS receiver may both be sent to a frequency counting circuit, whichdetermines, by means well-known in the art, the frequency ratio of thetwo reference signals. Since the frequency of the reference signal forthe GPS receiver has been determined, the frequency of the cellulartransceiver VCO can be determined from the frequency ratio. Since theVCO is phase or frequency locked to the carrier of the incomingbasestation signal, the carrier frequency can then be determined from asimple scaling procedure. In order to eliminate the Doppler frequencyoffset due to the motion of the mobile station relative to thebasestation, the location of the basestation is normally required inaddition to the velocity of the mobile. A server that performs the finalbasestation frequency calculation normally knows the location of thebasestation.

FIG. 4 shows an example of a cellular basestation which may be used withvarious embodiments of the present invention. The basestation 410includes a cellular transceiver 411 which connects to at least oneantenna 401 for communication signals to and from mobile cellularcommunication stations which are present in the area served by thecellular basestation 410. For example, mobile cellular communicationstations 210 and 310 may be mobile stations served by the cellularbasestation 410. The cellular transceiver 411 may be a conventionaltransceiver used to transmit and receive cellular signals, such as a GSMcellular signal or a CDMA cellular signal. Oscillator 413 may be aconventional system oscillator which controls the signal frequency ofthe basestation. The frequency of this oscillator may be measuredaccording to methods of the present invention for frequencysynchronization. In many cases oscillator 413 may be highly stable, butover a period of time, a small error in the frequency of the oscillatorwill cause the clock phase of the basestation to drift away from idealby a large amount. A precise measurement of the frequency of theoscillator can be used to predict the error in the clock of thebasestation and the error in the timing of the framing markerstransmitted by the basestation. Cellular basestation 410 typically alsoincludes a network interface 415 which transfers data to and from thecellular transceiver 411 in order to couple the cellular transceiver toa mobile switching center 421, as is well know in the art. The cellularbasestation 410 may also include a co-located data processing system423. Alternatively, data processing system 423 may be remote from thebasestation 410. In some embodiments, the data processing system 423couples to the oscillator 413 in order to adjust or recalibrate the timeof the clock to thereby synchronize the clock to other clocks in othercellular basestations according to methods of described in the copendingU.S. patent application Ser. No. 09/565,212, filed on May 4, 2000. Inmany cases the clock 413 is highly stable but freerunning and it wouldaffect network operation to actually alter the time epochs of the clock.Instead the time associated with the clock epochs can be adjusted. Thisis which is meant by “recalibration.” Hence, for the purpose offrequency synchronization, there may be no connection between the dataprocessing system 423 and oscillator 413. The data processing system 423is coupled to the network interface 415 in order to receive data fromthe cellular transceiver 411, such as time tag information for the framemarkers measured by the mobile systems for the purpose ofsynchronization to other cellular basestations, or for computing thefrequency of transmission of framing markers. In practice a basestationmay comprise a physical tower structure, one or more antennas and a setof electronics.

FIG. 5 shows an example of a data processing system which may be used asa server in various embodiments of the present invention. For example,as described in U.S. Pat. No. 5,841,396, the server may provideassistance data such as Doppler or other satellite assistance data tothe GPS receiver in the mobile station 210. In addition, oralternatively, the location server may perform the final positioncalculation rather than the mobile station 210 (after receivingpseudoranges or other data from which pseudoranges can be determinedfrom the mobile station) and then may forward this positiondetermination to the basestation so that the basestation may calculatethe frequency. Alternatively, the frequency may be calculated at thelocation server, or at other servers, or at other basestations. The dataprocessing system as a location server typically includes communicationdevices 512, such as modems or network interface, and is optionallycoupled to a co-located GPS receiver 511. The location server may becoupled to a number of different networks through communication devices512 (e.g., modems or other network interfaces). Such networks includethe cellular switching center or multiple cellular switching centers525, the land based phone system switches 523, cellular basestations,other GPS signal sources 527, or other processors of other locationservers 521.

Multiple cellular basestations are typically arranged to cover ageographical area with radio coverage, and these different basestationsare coupled to at least one mobile switching center, as is well known inthe prior art (e.g., see FIG. 1). Thus, multiple instances ofbasestation 410 would be geographically distributed but coupled togetherby a mobile switching center. The network 520 may be connected to anetwork of reference GPS receivers which provide differential GPSinformation and may also provide GPS ephemeris data for use incalculating the position of mobile systems. The network is coupledthrough the modem or other communication interface to the processor 503.The network 520 may be connected to other computers or networkcomponents such as the data processing system 423 in FIG. 4 (through anoptional interconnection not shown in FIG. 4). Also network 520 may beconnected to computer systems operated by emergency operators, such asthe Public Safety Answering Points which respond to 911 telephone calls.Various examples of method for using a location server have beendescribed in numerous U.S. patents, including: U.S. Pat. No. 5,841,396;No. 5,874,914; No. 5,812,087; and No. 6,215,442, all of which are herebyincorporated here by reference.

The location server 501, which is a form of a data processing system,includes a bus 502 which is coupled to a microprocessor 503 and a ROM307 and volatile RAM 505 and a non-volatile memory 506. Themicroprocessor 503 is coupled to cache memory 504 as shown in theexample of FIG. 5. The bus 502 interconnects these various componentstogether. While FIG. 5 shows that the non-volatile memory is a localdevice coupled directly to the rest of the components in the dataprocessing system, it will be appreciated that the present invention mayutilize a non-volatile memory which is remote from the system, such as anetwork storage device which is coupled to the data processing systemthrough a network interface such as a modem or Ethernet interface. Thebus 502 may include one or more buses connected to each other throughvarious bridges, controllers and/or adapters as is well known in theart. In many situations the location server may perform its operationsautomatically without human assistance. In some designs where humaninteraction is required, the I/O controller 509 may communicate withdisplays, keyboards, and other I/O devices.

Note that while FIG. 5 illustrates various components of a dataprocessing system, it is not intended to represent any particulararchitecture or manner of interconnecting the components as such detailsare not germane to the present invention. It will also be appreciatedthat network computers and other data processing systems which havefewer components or perhaps more components may also be used with thepresent invention.

It will be apparent from this description that aspects of the presentinvention may be embodied, at least in part, in software. That is, thetechniques may be carried out in a computer system or other dataprocessing system in response to its processor executing sequences ofinstructions contained in memory, such as ROM 507, volatile RAM 505,non-volatile memory 506, cache 504 or a remote storage device. Invarious embodiments, hardwired circuitry may be used in combination withsoftware instructions to implement the present invention. Thus, thetechniques are not limited to any specific combination of hardwarecircuitry and software nor to any particular source for the instructionsexecuted by the data processing system. In addition, throughout thisdescription, various functions and operations are described as beingperformed by or caused by software code to simplify description.However, those skilled in the art will recognize what is meant by suchexpressions is that the functions result from execution of the code by aprocessor, such as the processor 503.

In some embodiments the methods of the present invention may beperformed on computer systems which are simultaneously used for otherfunctions, such as cellular switching, messaging services, etc. In thesecases, some or all of the hardware of FIG. 5 would be shared for severalfunctions.

FIG. 6 shows a general system topology which may be used with thisinvention. The figure is very simplified for exemplary purposes;however, it illustrates a number of different situations that may beused in practice.

There are illustrated in FIG. 6 three mobile stations (615, 616, and617), two cellular basestations (613 and 614), a three satellite GPSconstellation (610, 611, and 612), and one location server 618.

Location server 618 communicates with other infrastructure via a(typically) wireline link 622, cellular infrastructure links 619 and 620(typically wireline), and a communication infrastructure 621 (typicallywireline). The emissions from the GPS satellites 623–625, areillustrated as with no fill. Those from basestation 613 have shading(e.g., 626); and those from basestation 614 have a solid fill (e.g.,627). The reception of signals by the mobile stations (with SPSreceivers) follows the same coding scheme. Thus, it is seen in FIG. 6that MS 615 receives signals from the GPS satellites and from BS 613; MS616 receives signals from the GPS satellites and from both BS 613 and BS614; and MS 617 receives signals from the GPS satellites and BS 614.

For simplicity all mobile stations receive signals from all GPSsatellites, although this is not necessary in practice. In practice,there may be a multiplicity of location servers, many more basestationsand mobile stations; and each individual mobile station may observeemissions from more than two basestations. Also, location servers may beco-located with the basestations or be remote from the basestation (asillustrated in FIG. 6).

In the example of FIG. 6 mobile station 616 would normally performtwo-way communications with only one of the basestations from which itreceives signals. For example, MS 616 may be performing two-waycommunications with basestation 613 and yet it may still receiveemissions from both basestations 613 and 614. Thus, MS 616 in this casemay perform synchronizing operations upon both basestations 613 and 614,although in this example, it would communicate synchronizationinformation only to basestation 613. It is well known in the art thatcellular telephones monitor other basestation emissions, in addition toa primary or “serving” site, in order to prepare for futurecommunications, or “handoffs”, to a different basestation.

FIG. 6 also shows a location server that may communicate data to andfrom mobile stations via a communication infrastructure and the cellularinfrastructure. The location server may be located at a basestation, buttypically is remote from the basestations, and in fact may communicatewith a number of basestations. The synchronization information providedby the mobile stations would typically be sent to one or more locationservers, which would process such information and determine the relativeor absolute timing of the transmissions of the basestations.

FIG. 7 shows the framing structure of traffic channels of GSM cellularsignals. In a GSM traffic signal, a superframe occurs every 6.12seconds; and a hyperframe occurs every 2048 superframes, or every 3.4816hours. Hence, the superframe is a useful epoch of granularity for timeinterval measurement. Alternatively, integral numbers of frames,multiframes, etc. may be used, since the times of their occurrence areuniquely defined as a multiple of bit durations.

In one embodiment of this invention, the duration of transmission ismeasured between two framing markers contained within a cellularcommunication signal transmitted by a cellular basestation. A set ofmeasurements is made by one or more mobile stations to determine theduration, i.e., timing of a later framing marker with respect to anearlier frame marker. The measured duration is compared (typically by aserver) to an expected timing. The result is used to determine the errorin the frequency of the basestation oscillator versus a desired value.

The error in measurement may be specified as a fraction of the truevalue, and be expressed in terms of parts per million (PPM). Forexample, if the time between specific framing markers is designed to be1 second but measured to be 1 second plus 1 microsecond, the error canbe expressed as 1 microsecond/1 second=1 PPM. This is the convenient wayto specify the error, since it also applies to the error of othersynchronized epochs (e.g., bit rate) as well as the error in carrierfrequency of the basestation, assuming (as is usually the case) that thetransmitted carrier frequency is synchronized to the framing markers.

Suppose one or more mobile stations measure the duration of abasestation signal corresponding to 98 transmitted superframes,approximately 10 minutes in time. The specific time of measurement canbe that corresponding to the beginning of a numbered multiframe. Themobile station keeps track unambiguously of the multiframe number bymeans of signaling information carried within the basebandtransmissions. Hence, the ideal period of measurement is exactly known,as expressed in units of transmitted bit duration (a bit period equals48/13 microseconds). The ideal measurement period is 98 times the idealperiod of each superframe, i.e., 599.76 seconds. However, the actualtime measurement is influenced by errors in the transmitter's clock, andby various measurement related errors.

When the duration between the two predetermined framing markers, lastingfor about 600 seconds, is measured with an error less than 1microsecond, the error in measured frequency of transmission of theframing markers is less than 0.00167 PPM. This precision is veryconsistent with the short term and long term frequency stability ofovenized crystal oscillators, which are commonly in use in cellularbasestations, even though the absolute accuracy of such oscillators isoften much poorer. In fact, in many cases the framing marker frequencymay be measured to a much greater precision. Although the maximumabsolute error in the frequency of GSM basestation reference oscillatorshas a specification of 0.05 PPM, the stability of these oscillations istypically much better than this specification.

The duration to be measured may be extended to over a period of evenhours to achieve a better accuracy in measurement, assuming that theshort term stability of the basestation oscillator supports suchaccuracy and that longer term drift characteristics (e.g., those due toaging) follow a smooth curve. As an example, a measurement periodextended to one hour with 1 microsecond precision implies that afrequency accuracy of 0.000278 PPM, which is again consistent with shortterm stability of good quality ovenized crystal oscillators. In fact itis common that the precision of good quality crystal oscillators is tentimes better than this.

Thus, measuring the duration of the period of transmission between twoframing markers using a mobile station can provide a very accuratemeasurement of the frequency of transmission of framing markers, whichmay be related to the frequency of the oscillator of the basestation.

FIG. 8 shows a flow chart for determining a frequency of a basestation'stransmissions according to one embodiment of the present invention. Inoperation 801, the arrival times of cellular signals transmitted by abasestation are measured at difference instances of time. The arrivaltimes of framing markers (e.g., boundaries of certain frames) aremeasured using one or more mobile stations (e.g., MS 210, MS 310, or MS615–617) with GPS receivers. Then, the frequency of transmission of thebasestation can be computed using the arrival times of these cellularsignals. The framing marker frequency can be calculated by dividing theknown numbers of framing markers present in the duration by theduration. Since the carrier frequency of the basestation signal and thefrequency of transmission of framing markers are synchronized to thefrequency of the main oscillator of the basestation, the frequency ofthe main oscillator of the basestation and the carrier frequency of thebasestation signal can be determined. In some embodiments it may becomputationally more convenient to compute the period of transmissionfrom the basestation.

As stated earlier, the determination of cellular transmitter frequencywould typically be done at a server, or so-called Position DeterminationEntity (PDE), rather than at a cellular basestation, although the PDEmay be collocated with the cellular basestation. This server or PDE is aset of equipment that resides in the cellular or communication networkinfrastructure which may pass messages to and from the mobile stationsvia communication networking, cellular networking and wireless links.That is, once the mobiles make timing related measurements of thebasestation transmissions, such measurements are transmitted over thecellular link to a serving basestation and then via infrastructure landlines to the PDE. The PDE would then utilize these measurements tocompute the time and frequency associated with future framing markers.This information may then be passed to the mobiles or to other networkentities wishing to utilize such information to improve systemperformance. In fact, in one embodiment, such timing information acts asassistance data that allows the mobile stations to perform future GPSreception and measurement operations in a more efficient manner. Thisembodiment then provides a “bootstrap” approach where prior GPSmeasurement performed by some mobiles greatly aid the performance oflater GPS measurements. Performance enhancements in this manner includegreatly increased sensitivity, reduced time to first fix, and increasedavailability, as described in U.S. Pat. No. 5,841,396, and No.5,945,944.

FIG. 9 shows a detailed method to determine a frequency of basestationsignals by using measurements of framing epochs of the basestationsignals according to one embodiment of the present invention. Inoperation 901–909, a first mobile station (MS) receives a cellularsignal from a basestation (BS); finds a framing marker contained withinsuch cellular signal; finds the time of day and its own location usingits GPS receiver; assigns a time tag to the framing markers using thetime-of-day found in operation 905; and sends its location (orinformation for the determination of its location) and the time tags (orinformation for the determination of the time tags) to a server, such asa location server.

It will be appreciated that operation 905 may precede operations 901 and903, or be concurrent with operations 901 and 903. The transmission pathfor sending the location and time tag information typically includes acellular link followed by additional terrestrial links (e.g., telephonelines, local area networks, etc).

The cellular signal received in operation 901 may be over a differentcommunication link than that used to transmit the data in operation 909.That is, the basestation observed in operation 901 may not be the“serving” basestation for the mobile station. It may be one that themobile station briefly observes to determine a “neighbor” list ofbasestations, which might be used at a later time during a handoffoperation. It is often the case that a mobile station may observe asmany as 10 basestations or more, as is well known in the art.

A second mobile station (or even the same basestation) performsoperations 911–919 in a manner similar to operations 901–909. Typically,operations 911–919 are performed at a different instance of time otherthan when operations 901–909 are performed. It will be appreciated thatoperations 911–919 may be performed by the same mobile station thatperformed operation 901–909, but at a different instance of time.

In operation 921, the server (e.g., a location server) processes thetime tags received from the mobile stations, the locations of the mobilestations, and the information about the basestation location to computea frequency related to the basestation, such as a frequency associatedwith the framing marker rate or any other frequencies of the basestationthat are synchronized to this rate. The frequency may be expressed interms of a nominal (ideal or theoretical) frequency and an error, withthe latter expressed in dimensionless PPM units, for example. Since thetime tags correspond to the instances of times when the framing markersarrived at the measuring mobile station (or stations), the locations ofthe mobile stations and the basestation are needed to convert the timetags into time measures at a same location in order to compute a preciseduration of the transmission. This is done by subtracting from the timetags the delays for the cellular signal to travel from the transmittingbasestation to the measuring mobile stations.

In operation 923, the times-of-occurrence of future basestation framingmarkers can be predicated using the measured frequency of transmission.Such predictions may be transmitted to various network entities such asbasestations or mobile stations upon request in operation 925.

Since the information provided to the server in operations 909 and 919also allows the determination of the time-of-day associated with theframing markers, time coordination may also be performed according tothe methods described in the co-pending U.S. patent application Ser. No.09/565,212, filed on May 4, 2000.

In operation 927, the predicted epoch timing can be used by mobilestations or basestations for aiding SPS measurements or TDOA or EOTDoperations.

While the FIG. 9 illustrated a method to determine the frequency oftransmission of a basestation using two mobile stations and onebasestation, in practice, there may typically be many more mobilestations involved. In addition, each mobile station may simultaneouslyor sequentially view the timing epochs of several basestations. Hence,multiple operations like operations 901–909 (or 911–919) may take placein parallel corresponding to multiple basestations. The processing asshown in FIG. 9 may proceed on a continuing basis. As mentioned earlier,the operations of FIG. 9 may be carried out by a single mobile stationobserving one or more basestations.

The errors in the epoch predictions may be reduced by modeling the longterm frequency versus time (drift) characteristics of the basestation.In many situations the long term drift is smooth and fairly predictablefor good quality basestation oscillators. Thus, the driftcharacteristics can be determined from multiple measurements ofbasestation transmissions over very long periods of time. A curvefitting procedure can be used to predict future drifts from the driftcharacteristics. Typical curve fitting algorithms may use polynomials.

In the method as shown in FIG. 9, it is not necessary that the samemobile station make the subsequent timing measurements. In fact, each ofthe timing measurements, corresponding to a given basestation, may bemade by different mobile stations. When a large number of measurementsare made over a period of time, various averaging operations, such asleast-mean square (LMS) averaging, may be performed. Processing a largenumber of measurements not only reduces the measurement errorsignificantly, but also permits the discarding of measurements which maycontain unusually high errors due to spurious effects, such as multipathreception of the basestation transmissions. Such discarding of“outliers” may be done by first making an initial estimate of thefrequency using all measurements, then discarding those measurementsthat appear to be well off this initial measurement, and finallyre-computing the estimate using the measurements that have not beendiscarded. Other approaches, such as those using order statistics, mayalso be used to discard outliners.

The cellular signal arriving at a mobile station may be a result ofreflection of the primary signal or the presence of multiple direct andreflected received signals, so-called “multipath”. In most cases,multipath results in a positive excess delay, i.e., a longer delay insignal transmission than that in a direct line-of-sight transmission.The delay for line-of-sight transmission can be determined by dividingthe distance between the basestation and mobile station by the speed oflight. Since it is rare that multipath produces a negative excess delay,simple averaging may not be the best approach to reduce the error due tomultipath.

The excess delay due to multipath may be compensated by using weightedaveraging. One method is to select, or heavily weight, the measurementsthat are derived from high quality signals, for example, signals of highstrength (high signal-to-noise ratios) and signals with narrow,well-defined signal shapes. Some type of autocorrelation analysis toanalyze the received signal shape may be used to determine the qualityof the received signal. High quality signals tend to result more oftenfrom line-of-sight transmission, or from situations with minimalreflections, and hence exhibit less excess delays than low qualitysignals. In some situations, with a sufficiently high received signallevel, it is possible to utilize signal processing algorithms toestimate the number, strengths, and relative delays of the receivedsignals from a given basestation. In this case the smallest delay may bechosen in order to minimize the effect of excess delay.

While FIG. 9 illustrates a method where the duration of transmission iscomputed at a server, FIG. 10 shows another method where the duration oftransmission is determined at a mobile station. In operations 1001–1007,a mobile station receives a cellular signal from a basestation (BS) andthe BS location; finds a framing marker contained within such cellularsignal; finds its location and the time of day using its GPS receiver;and assigns a time tag to the framing markers using the time-of-dayfound in operation 1005. Similarly, a time tag for a second framingmarker is determined in operations 1011–1017. In operation 1019, themobile station computes the duration of the transmission time using thetime tags. In this case, information about the position of the mobilestation and the basestation are typically required, since the mobilestation may have moved between measurements and hence the change inbasestation-mobile range must be compensated. If it is known that themobile is stationary, then this information is not required. Thefrequency of transmission of framing markers for the basestation can bedetermined and can be used to predict the timing of future framingmarkers of the basestation. The duration or the measured frequency maybe transmitted to a server, and the prediction of the timing may beperformed on a server. In operations 1022 and 1023, the prediction canbe provided to mobile stations or basestations for aiding in SPSmeasurement, or in EOTD or TDOA operations. The first and secondcellular signals in FIG. 10 typically correspond to two portions of thecellular signal received at different times during the same telephone“call”. However, these may also correspond to signals from thebasestation received during separate calls.

FIG. 11 shows a detailed method to determine a frequency of basestationsignals by using measurements of carrier frequency of the basestationsignals according to one embodiment of the present invention. Inoperation 1101, a mobile station receives a cellular signal transmittedfrom a basestation. It synchronizes to the carrier frequency of thereceived cellular signal in operation 1103. This is normally done usingeither a Phase-Locked Loop (PLL) or Automatic Frequency Control (AFC)circuit, either of which contains a voltage-controlled oscillator (e.g.,VCO 323). The synchronization procedure causes the VCO to bear aproportional relationship to either the phase or frequency of thereceived carrier.

In operation 1105 the mobile station uses a GPS (or SPS) receiver todetermine its location, velocity, the time-of-day, and the frequency ofthe reference signal from its local oscillator. For the determination ofa frequency of the basestation, the measurement of the frequency of thelocal oscillator reference is the primary information of interest;however the location, velocity and time-of-day information are typicalbyproducts of the GPS processing. The location and velocity are requiredto determine the effect of the MS motion on the frequency measurement.As discussed previously, the local reference signal used by the GPSreceiver may be provided by the VCO of the cellular transceiver or maybe provided by a separate crystal oscillator.

In operation 1107 the mobile station determines the received basestationcarrier frequency from the VCO signal and from the GPS referencefrequency measurement. As described earlier, this is a direct byproductof the GPS processing if the VCO is used as its frequency reference.Alternatively, separate frequency counting circuitry may be utilized todetermine the frequency ratio of the VCO and GPS reference signals. Thefrequency ratio and the value of the GPS reference frequency, determinedwhile processing GPS signals, provide a precise estimate of the VCOfrequency and hence the carrier frequency of the received basestationsignal.

In operation 1109, the frequency information is sent, with the auxiliarydata (e.g., time-of-day, basestation identity information, and others)to a server. In operation 1111, the carrier frequency information, whichmay be expressed in PPM units or other units, may be used to compute thebasestation oscillator frequency, and/or other frequencies (e.g.,framing mark frequency). The location and velocity of the mobile areused together with the basestation location to determine the frequencyerror due to the mobile-basestation relative motion. This error must beremoved in order to get an accurate estimate of the frequency of thebasestation. The server might combine a number of such frequencymeasurements together to further improve the estimate of basestationfrequency. Finally, in operations 1113–1117, the server predicts thetiming of future basestation marker epochs from this frequencyinformation and sends it to other network elements (e.g., mobilestations, or basestations, or location servers) upon request for aidingmeasurements (e.g., SPS measurements, or TDOA or EOTD operations).

While FIG. 11 illustrations a scenario involving only a mobile stationand a basestation, in practice, there may be many more mobile stationsinvolved. Each mobile station may simultaneously or sequentially viewthe transmissions of several basestations. Hence, multiple sequences ofoperations (as operations 1101–1109) may take place in parallelcorresponding to multiple basestations. It will also be appreciated thatthe processing as shown in FIG. 11 may proceed on a continuing basis.

A number of other variations to the methods of FIGS. 8–11 should beapparent to those skilled in the art. For example, the mobile stationmay perform the calculations 1111–1117 if it receives the location ofthe basestation. In FIG. 10 instead of measuring time-of-day inoperations 1005 and 1015, the mobile may compute the elapsed time afterit has calibrated its clock via the method of 1101–1107 of FIG. 11.

When the basestation oscillator is sufficiently stable, the basestationfrequency calibration can allow the accurate prediction of epochs offuture timing markers transmitted by the basestation. Typically, thestability of basestation oscillator is sufficient to allow accuratetiming predictions over very long periods of time, once a timecoordination is performed.

The basestations typically utilize high quality ovenized crystaloscillators as frequency references. Some basestations further locktheir references to transmitted signals from GPS satellites, in whichcase the long term stability of the basestation transmissions would belocked to Cesium type stability, and be suitable for accurate timingpredictions. In the following discussions, we assume that such GPSlocking is not utilized. In this case the two major sources ofbasestation oscillator instability are: i) short term frequencyinstability which is usually characterized by short term frequencystability measures such as noise spectral density methods or AllanVariance; and ii) longer term frequency drift which is typicallyassociated with aging effects. Long term frequency drift tends to be onthe order of 0.001 PPM per day or better and hence should not representa significant source of error over relatively short periods of time(e.g., 15 to 30 minutes).

Most basestation oscillators utilize ovenized crystal oscillators. Smallchanges in temperature of the oven or voltage supplied to the oven cancontribute to increases in frequency error. In addition certain shortterm frequency stability characteristics, such as Random-walk frequencyeffects, produce a frequency error that grows as a function ofobservation time [see J. Rutman and F. L. Walls, Characterization ofFrequency Stability in Precision Frequency Sources, Proc. IEEE, Vol. 79,No. 6, June 1991, pp. 952–959]. Thus, it is important to examine themagnitude of these effects both from a device and a system standpoint.

The short term frequency stability considered herein is that measuredover a time interval of several seconds to several hours. Measured overthese periods good quality ovenized oscillators typically have shortterm stability (fractional frequency deviation, or so-called Allanvariance) on the order of 0.00001 PPM. With this stability the timingsignals from a basestation may be predicted over a future period of 10minutes to an accuracy of 6 nanoseconds and over a future period of 1hour to an accuracy of 36 nanoseconds.

The long term stability of good quality ovenized oscillators may be onthe order of 0.001 PPM per day or better, corresponding to around0.00004 PPM per hour [see Fundamentals of Quartz Oscillators, HewlettPackard Application Note 200-2]. Thus, for predictions over a period ofthe order of an hour or more, the effects from aging characteristics candominate.

From a measurement standpoint, Pickford considered the frequency driftbetween two basestations, based upon the use of round trip measurements[see Andrew Pickford, BTS Synchronization Requirements and LMU UpdateRates for E-OTD, Technical Submission to Technical Subcommittee T1P1,Oct. 8, 1999]. He found that once a linear phase (or time) drift (i.e.,fixed frequency offset error) was removed, the net RMS time error was onthe order of 66 nanoseconds even for periods exceeding 1 hour. He alsodemonstrated that utilizing measurements over a 1 hour period andprojecting them forward for the next hour yielded similar accuracy.Furthermore, an examination of his curves indicated that the residualerror after removing the average drift was dominated by what appeared tobe random errors. This might indicate that the predominant remainingerrors were due to measurement errors, or additive noise, rather thanactual oscillator jitter. Note that an error of 66 nanoseconds RMS,measured over an hour period, is equivalent to a frequency stability ofaround 0.000018 PPM, which is typical of a good quality crystaloscillator.

Another similar paper of T. Rantallainen, et. al., provided similarresults to the above [see T. Rantallainen and V. Ruutu, RTD Measurementsfor E-OTD Method, Technical Submission to T1P1.5/99-428R0, Jul. 8,1999]. However, in this paper several of the fits to phase vs. timerequired a second order polynomial in order to keep the residual errorslow. Typical time intervals over which processing was done ranged fromabout 1500 to 2200 seconds. An explanation was not given for thenonlinear characteristic of the phase versus time plot. This may verywell be due to aging characteristics of the crystal oscillator, asindicated above. Since aging characteristics tend to be predictable andsmooth, the polynomial fit algorithm should work well. For example, asecond order polynomial fit to frame period versus measurement time willcompensate for a linear frequency versus time drift.

Additional factors that can contribute to small changes in frequencyversus time include voltage and temperature fluctuations of thefrequency references. These factors can manifest themselves as verysmall frequency changes. Basestations tend to have regulated voltagesand temperatures in order to ensure high reliability.

When there is significant user motion, it is important that any Dopplerrelated effects do not unduly influence the timing and frequencymeasurements described above. In particular, if the mobile stationmeasures time at one instance and predicts the time-of-day associatedwith a cellular signal frame boundary occurring at a different instance,an error can result from the motion of the mobile station, especially ifthe mobile is rapidly moving and/or the difference between these timeinstances is large. There are a number of ways to deal with this type ofproblems. For example, when the mobile station can determine itsvelocity, the data about the velocity of the mobile station may besupplied to the server in order to compensate for the errors due to theDoppler effects associated with the range rate between the mobile andthe basestation. This approach has been shown in FIG. 11. As describedabove, the GPS signals can be processed to estimate the velocity of thereceiving platform. This information may be utilized to compensate forany errors due to the motion of the mobile station.

Some residual errors may remain, such as multipath delays and transitdelays through the mobile station hardware. However, the mobile stationand/or basestation can often determine the degree of such degradationsand weight those measurements more heavily that have less error.

The effective times of transmission (i.e., the arrival time) aredetermined at the face of the basestation antennas. The use of a largenumber of mobile stations may tend to reduce errors via averagingprocedures. This assumes that system biases may be eliminated or reducedby appropriate measurement selection or other bias estimationprocedures.

Concerns about sufficient mobile station activity to support the timing(e.g. early morning hours) could be ameliorated by placing mobilestations at various locations and making calls periodically. However,these need not be fixed assets.

Typical timing errors due to the GPS processing at a single mobilestation might be on the order of 10–30 nanoseconds. Thus, other sourcesof error, such as multipath may dominate.

The stability of the basestation oscillator affects how often timingmeasurements need to be made and disseminated. It is possible by use ofa multiplicity of measurements from mobile stations to preciselydetermine not only the instantaneous frequency of the basestationoscillator, but also higher moments such as the rate of change of suchfrequency. As discussed above, it is normally the case that a simplecurve fit to the basestation frequency versus time may be maintained toextremely high accuracy over long periods of time.

Although the methods and apparatus of the present invention have beendescribed with reference to GPS satellites, it will be appreciated thatthe teachings are equally applicable to positioning systems whichutilize pseudolites or a combination of satellites and pseudolites.Pseudolites are ground based transmitters which broadcast a PN code(similar to a GPS signal) modulated on an L-band carrier signal,generally synchronized with GPS time. Each transmitter may be assigned aunique PN code so as to permit identification by a remote receiver.Pseudolites are useful in situations where GPS signals from an orbitingsatellite might be unavailable, such as tunnels, mines, buildings orother enclosed areas. The term “satellite”, as used herein, is intendedto include pseudolite or equivalents of pseudolites, and the term GPSsignals, as used herein, is intended to include GPS-like signals frompseudolites or equivalents of pseudolites.

In the preceding discussion the invention has been described withreference to application upon the United States Global PositioningSatellite (GPS) system. It should be evident, however, that thesemethods are equally applicable to similar satellite positioning systems,and in particular, the Russian Glonass system and the proposed EuropeanGalileo System. The Glonass system primarily differs from GPS system inthat the emissions from different satellites are differentiated from oneanother by utilizing slightly different carrier frequencies, rather thanutilizing different pseudorandom codes. In this situation substantiallyall the circuitry and algorithms described previously are applicable.The term “GPS” used herein includes such alternative satellitepositioning systems, including the Russian Glonass system and theproposed European Galileo System.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

1. A method to measure a frequency related to a basestation of acellular communication system, the method comprising: receiving, at amobile station, at least one satellite positioning system signal;determining a frequency of a reference signal from a local oscillator ofthe mobile station from the at least one satellite positioning systemsignal; receiving, at the mobile station, a first cellular signal fromthe basestation, the first cellular signal containing a first timingmarker and a second timing marker; determining a first time tag for thefirst timing marker and a second time tag for the second timing marker,using the reference signal from the local oscillator; and combining thefrequency of the reference signal from the local oscillator with thefirst and second time tags to compute a first frequency related to thebasestation.
 2. A method as in claim 1 further comprising: transmitting,through a communication link, the first frequency to a server.
 3. Amethod as in claim 1 wherein said combining further comprises: computinga time difference between the time tags.
 4. A method as in claim 1wherein the first frequency is related to a carrier frequency of asignal from the basestation.
 5. A method as in claim 1 wherein the firstfrequency is related to a symbol rate of a signal from the basestation.6. A system to measure a frequency related to a basestation, the systemcomprising: a mobile station comprising: a cellular transceiverconfigured to receive from the basestation a cellular signal containinga first timing marker and a second timing marker; a local oscillatorgenerating a reference signal; a satellite positioning system receivercoupled to the local oscillator, the satellite positioning systemreceiver configured to receive at least one satellite positioning systemsignal and to determine a frequency of the reference signal from the atleast one satellite positioning system signal; and a processor coupledto the cellular receiver and the satellite positioning system receiver,the processor configured to determine a first time tag for the firsttiming marker and a second time tag for the second timing marker usingthe reference signal and to combine the frequency of the referencesignal with the first and second time tags to compute a first frequencyrelated to the basestation.
 7. A system as in claim 6 wherein thesatellite positioning system receiver is configured to determinelocation and velocity data of the mobile station using the at least onesatellite positioning system signal.
 8. A system as in claim 6 furthercomprising: a server coupled to the mobile station through acommunication link, the mobile station transmitting through thecommunication link the first frequency to the server using the cellulartransceiver.
 9. A system as in claim 6 wherein the satellite positioningsystem receiver and the cellular transceiver share at least one commoncomponent.